The present invention relates to a magnetic recording medium comprising a non-magnetic support on which a magnetic layer containing a ferromagnetic powder dispersed in a binder is provided via a non-magnetic lower layer. More specifically, the invention is concerned with a magnetic recording medium which has excellent error-rate characteristic and high durability and is suitable for record and playback of digital signals, especially suitable as magnetic tape.
Magnetic recording media which each have on a non-magnetic support a magnetic layer containing a ferromagnetic powder dispersed in a binder are widely used as sound recording tapes, videotapes or floppy disks. And it is required for these recording media that their characteristics, such as electromagnetic conversion characteristic, running durability and other running properties, be on high levels. For instance, higher level of ability to reproduce original sounds is required of audio tapes for reproduction of musical recordings. And excellent electromagnetic characteristics, such as excellent ability to reproduce original pictures, are required for video tapes.
In order to achieve excellent electromagnetic characteristics adaptable to the aforesaid requirements, the shift from xcex3-iron oxide to metallic magnetic substances as a ferromagnetic powder contained in the magnetic layer is pushed as part of improvement, and thereby enhancement of coercive force Hc and saturation magnetization "sgr"s are performed. In particular, the shift to metallic magnetic substances is pushed favorably in the cases of 8-mm videotapes and videotapes for recording pictures in the broadcasting field. In addition, there has been a growing trend in recent years to record and reproduce picture and music in digital form, and degradation that original pictures or sounds experience in their reproduction and edit processes has been eliminated to enable faithful reproduction of original pictures or sounds. For evaluating tape performance in digitized settings, the error rate at the time of reproduction (rate of code error) is used. In evaluating the error rate, Japanese Patent No. 2,829,972 proposes the method of making a separation between the instrument-originated error rate and the tape-originated error rate. And in order to improve a tape-originated error rate, it has been studied to smoothen the tape surface, to make a magnetic layer be highly packed, to increase output by heightening Hc, or to elevate CNR. As the recording density is increased, however, the feasibility of improving the error rate by merely increasing output or CNR is decreased. In particular, such a tendency predominates in the cases of using digital recording wavelengths of 0.5 xcexcm or below. As to the improvement in error rate, JP-A-11-175959 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent application) proposes magnetic recording media for achieving a favorable error rate characteristic. However, the magnetic recording media as described in Examples of the related art are insufficient in still durability and error rate characteristic at recording frequencies of 7 MHz or above. In addition, JP-B-3-45447 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d specifies magnetic characteristics and orientation degree of tape. However, sufficient output cannot be attained so long as the magnetic layer thickness is within the scope of Examples.
In view of the past situation as mentioned above, the invention aims to provide a magnetic recording medium having satisfactory error rate and durability when it is applied to a record reproduction apparatus operating at frequencies of 10 MHz or above.
The aforesaid objective is attained with the following magnetic recording media as embodiments of the invention.
(1) A magnetic recording medium comprising a non-magnetic support on which a magnetic layer containing a ferromagnetic powder dispersed in a binder is provided via a non-magnetic lower layer: with the magnetic layer having a coercive force of 2,500 to 3,500 oersted (197.5 to 276.5 kA/m) and a squareness ratio of 0.70 to 0.85 in the length direction, wherein the binder in the magnetic layer is a urethane resin having a glass transition temperature Tg of 70xc2x0 C. or higher.
(2) A magnetic recording medium as described in Embodiment (1), wherein the ferromagnetic powder is an Fe-dominated ferromagnetic powder which further contains Co in a proportion of 30 to 40 atomic % to the Fe, either Al or Si or both of them in a proportion of 2 to 20 atomic % to the Fe, and either Y or Nd or both of them in a proportion of 7 to 15 atomic % to the Fe, and has a specific surface area of no greater than 80 m2/g as measured by BET method.
(3) A magnetic recording medium as described in Embodiment (1) or (2), wherein the magnetic layer has a thickness of 0.05 to 0.3 xcexcm.
(4) A magnetic recording medium as described in any of Embodiments (1) to (3), wherein the lower layer has a thickness of 0.8 to 2.0 xcexcm.
(5) A magnetic recording medium as described in any of Embodiments (1) to (4), which has a maximum magnetic flux density Bm of no greater than 3,800 Gauss and contains polyurethane having a glass transition temperature Tg of 70xc2x0 C. or above in both the lower layer and the magnetic layer.
As a result of our intensive study to accomplish the aforementioned objective, the following finding has been made and the invention has been completed on the basis of this finding. Specifically, it has been found that reduction of error rate cannot be attained by a mere increase of output. More specifically, the finding is that, even if calender treatment is performed at the time of tape production with the intention of heightening an orientation force, achieving a high squareness ratio (SQ) and attaining the desired surface properties and thereby the magnetic layer is highly packed and Bm is increased to enable the magnetic recording medium to acquire high output, a deterioration in error rate is caused at recording frequencies higher than 20 MHz because of degradation in wave-form responsivity as disclosed in JP-A-5-28464. In order to avoid the deterioration in error rate, we have made various examinations and found that there is the most suitable squareness ratio for effecting both attainment of high output and reduction of error rate. Further, it has been found that adjustment of a coercive force Hc of the magnetic layer to the range of 2,500 to 3,500 oersted (197.5 to 276.5 kA/m) is essential for the objective of the invention to be attained. The term xe2x80x9cHcxe2x80x9d as used herein refers to the coercive force value in the tape-running direction. When Hc is lower than 2,500 oersted (197.5 kA/m), the output at short wavelengths cannot be secured; while when Hc is higher than 4,000 oersted (316 kS/m) saturation of a head used for recording is caused to disable the attainment of high output. Therefore, the suitable Hc is from 2,500 to 3,500 oersted (197.5 to 276.5 kA/m), preferably from 2,700 to 3,000 oersted (213.3 to 237 kA/m). To sum up, the invention achieves a reduction in error rate while ensuring high output by combining the Hc setting of the magnetic layer within the specified range and the optimization of a squareness ratio of the magnetic layer. In addition, it has been found that improvement in still durability of a magnetic recording medium can be effected by combining the optimization of a squareness ratio of the magnetic layer and the optimization of the lower layer thickness. Such improvement effect is thought to be attributable to reduction in sliding resistance of a head to the magnetic layer surface brought about by the supply of a lubricant to the magnetic layer surface and the spread of an angle distribution of magnetic grains in the length direction and the spread of an angle distribution of magnetic grains in the cross-sectional direction of the magnetic layer.
Furthermore, it has been found that the adjustment of a coercive force of the magnetic layer to the range of 2,500 to 3,500 oersted can be adequately attained by using as the magnetic powder contained in the magnetic layer a Fe-dominated ferromagnetic powder which further contains Co in a proportion of 30 to 40 atomic % to the Fe, either Al or Si or both of them in a proportion of 2 to 20 atomic % to the Fe, and either Y or Nd or both of them in a proportion of 7 to 15 atomic % to the Fe. In addition, it has been also found that, when the ferromagnetic powder used in a magnetic coating composition has a specific surface area of no greater than 80 m2/g as measured by BET method, the magnetic layer surface formed by coating the magnetic coating composition, subjecting the coating layer to calendering treatment and forming tapes therefrom can be made smooth to ensure a further increase of output.
The value of a coercive force Hc as specified above is a value measured by using a vibration sample magnetometer (made by Toei Kogyo K.K.) under a condition of Hm=10 kOe (790 kA/m). Moreover, the optimal range of the squareness ratio of the present magnetic layer is from 0.7 to 0.85, especially from 0.75 to 0.80. And it is appropriate that the squareness ratios in two directions perpendicular to the tape-running direction be each not higher than 80% of the squareness ratio in the running direction.